Ferroelectricity in Li-implanted ZnO thin films

ZnO:Li thin films were prepared by implantation of ZnO with a Li ion fluence 5 × 1016 ions/cm2 at implantation energies of 50, 100, 200 keV. Ferroelectric characterization of the implanted samples revealed a clear hysteresis in the polarization-field curves. The origin of the ferroelectricity can be attributed to an off-center dipole caused by the large difference in ionic radii between the host Zn (0.74 Å) and the dopant Li (0.60 Å). ZnO:Li films which were implanted at 200 keV and annealed at a temperature of 700 °C exhibited a well-defined polarization hysteresis loop, with a remanent polarization of 0.8 μC/cm2 and coercive field of 8.2 kV/cm, at room temperature. The dielectric phase transition was observed in the temperature range from 340 to 360 K. It is concluded that this novel ferroelectric phase transition in ZnO:Li results from the small structural distortion induced along the c-axis.